In the conventional method of making electrophotographic toner powders, a binder polymer and other ingredients, such as a pigment and a charge control agent, are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrophotographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrophotographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is, therefore, higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
The preparation of toner polymer powders from a preformed polymer by the process known as “evaporative limited coalescence” offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al., incorporated herein by reference for all that they contain.
This technique includes the following steps: mixing a polymer material, a solvent and optionally a colorant and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent and washing and drying the resultant product.
Polymeric powders can also be prepared by emulsion and suspension polymerization techniques. In suspension polymerization, polymerization initiator and additives such as a colorant are dissolved into water-insoluble monomers, and the resulting composition is suspended under high-speed shear stirring into an aqueous solution comprising an appropriate dispersant, for example, a water-soluble polymer, an inorganic powder and a surface active agent, and the suspension is subjected to polymerization to form colored polymer particles. The solidified polymer particles are separated from the remainder of the system. Patents utilizing these techniques to prepare toner polymer particles having a narrow size distribution referred to as “limited coalescence polymerization.” Several examples in numerous patents include U.S. Pat. Nos. 2,934,530; 3,615,972; 2,932,629 and 4,314,932 and are incorporated by reference herein.
There is a need to reduce the amount of toner applied to a substrate in the electrophotographic process (EP). Porous toner particles in electrophotographic process are supposed to reduce the toner mass in the image area. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Hence, toner particles having an elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost of the print and improve the print quality.
An object of the present invention is to provide a toner particle with increased porosity.